What are some reference words?

Hi guys,
I hesitated asking this question, but I really need some help
By reference words, I mean words like
”According to the National Cancer Institute…”
”...reports the American Cancer Society”
What are some more words like that? In my last English class, there were a whole bunch of them, but I can’t remember.
I’m writing a term paper on cancer, and I don’t want to keep using “According to…” over and over again.

Thank you :)